More than 200 million people worldwide have diabetes. The World Health Organization estimates that 1.1 million people died from diabetes in 2005 and projects that worldwide deaths from diabetes will double between 2005 and 2030. New chemical compounds that effectively treat diabetes could save millions of human lives.
Diabetes refers to metabolic disorders resulting in the body's inability to effectively regulate glucose levels. Approximately 90% of all diabetes cases are a result of type 2 diabetes whereas the remaining 10% are a result of type 1 diabetes, gestational diabetes, and latent autoimmune diabetes of adulthood (LADA). All forms of diabetes result in elevated blood glucose levels and, if left untreated chronically, can increase the risk of macrovascular (heart disease, stroke, other forms of cardiovascular disease) and microvascular [kidney failure (nephropathy), blindness from diabetic retinopathy, nerve damage (diabetic neuropathy)] complications.
Type 1 diabetes, also known as juvenile or insulin-dependent diabetes mellitus (IDDM), can occur at any age, but it is most often diagnosed in children, adolescents, or young adults. Type 1 diabetes is caused by the autoimmune destruction of insulin-producing beta cells, resulting in an inability to produce sufficient insulin. Insulin controls blood glucose levels by promoting transport of blood glucose into cells for energy use. Insufficient insulin production will lead to decreased glucose uptake into cells and result in accumulation of glucose in the bloodstream. The lack of available glucose in cells will eventually lead to the onset of symptoms of type 1 diabetes: polyuria (frequent urination), polydipsia (thirst), constant hunger, weight loss, vision changes, and fatigue. Within 5-10 years of being diagnosed with type 1 diabetes, patient's insulin-producing beta cells of the pancreas are completely destroyed, and the body can no longer produce insulin. As a result, patients with type 1 diabetes will require daily administration of insulin for the remainder of their lives.
Type 2 diabetes, also known as non-insulin-dependent diabetes mellitus (NIDDM) or adult-onset diabetes, occurs when the pancreas produces insufficient insulin and/or tissues become resistant to normal or high levels of insulin (insulin resistance), resulting in excessively high blood glucose levels. Multiple factors can lead to insulin resistance including chronically elevated blood glucose levels, genetics, obesity, lack of physical activity, and increasing age. Unlike type 1 diabetes, symptoms of type 2 diabetes are more salient, and as a result, the disease may not be diagnosed until several years after onset with a peak prevalence in adults near an age of 45 years. Unfortunately, the incidence of type 2 diabetes in children is increasing.
The primary goal of treatment of type 2 diabetes is to achieve and maintain glycemic control to reduce the risk of microvascular (diabetic neuropathy, retinopathy, or nephropathy) and macrovascular (heart disease, stroke, other forms of cardiovascular disease) complications. Current guidelines for the treatment of type 2 diabetes from the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) [Diabetes Care, 2008, 31 (12), 1] outline lifestyle modification including weight loss and increased physical activity as a primary therapeutic approach for management of type 2 diabetes. However, this approach alone fails in the majority of patients within the first year, leading physicians to prescribe medications over time. The ADA and EASD recommend metformin, an agent that reduces hepatic glucose production, as a Tier 1a medication; however, a significant number of patients taking metformin can experience gastrointestinal side effects and, in rare cases, potentially fatal lactic acidosis. Recommendations for Tier 1b class of medications include sulfonylureas, which stimulate pancreatic insulin secretion via modulation of potassium channel activity, and exogenous insulin. While both medications rapidly and effectively reduce blood glucose levels, insulin requires 1-4 injections per day and both agents can cause undesired weight gain and potentially fatal hypoglycemia. Tier 2a recommendations include newer agents such as thiazolidinediones (TZDs pioglitazone and rosiglitazone), which enhance insulin sensitivity of muscle, liver and fat, as well as GLP-1 analogs, which enhance postprandial glucose-mediated insulin secretion from pancreatic beta cells. While TZDs show robust, durable control of blood glucose levels, adverse effects include weight gain, edema, bone fractures in women, exacerbation of congestive heart failure, and potential increased risk of ischemic cardiovascular events. GLP-1 analogs also effectively control blood glucose levels, however, this class of medications requires injection and many patients complain of nausea. The most recent addition to the Tier 2 medication list is DPP-4 inhibitors, which, like GLP-1 analogs, enhance glucose-medicated insulin secretion from beta cells. Unfortunately, DPP-4 inhibitors only modestly control blood glucose levels, and the long-term safety of DPP-4 inhibitors remains to be firmly established. Other less prescribed medications for type 2 diabetes include α-glucosidase inhibitors, glinides, and amylin analogs. Clearly, new medications with improved efficacy, durability, and side effect profiles are needed for patients with type 2 diabetes.
GLP-1 and GIP are peptides, known as incretins, that are secreted by L and K cells, respectively, from the gastrointestinal tract into the blood stream following ingestion of nutrients. This important physiological response serves as the primary signaling mechanism between nutrient (glucose/fat) concentration in the gastrointestinal tract and other peripheral organs. Upon secretion, both circulating peptides initiate signals in beta cells of the pancreas to enhance glucose-stimulated insulin secretion, which, in turn, controls glucose concentrations in the blood stream (For reviews see: Diabetic Medicine 2007, 24(3), 223; Molecular and Cellular Endocrinology 2009, 297(1-2), 127; Experimental and Clinical Endocrinology & Diabetes 2001, 109(Suppl. 2), S288).
The association between the incretin hormones GLP-1 and GIP and type 2 diabetes has been extensively explored. The majority of studies indicate that type 2 diabetes is associated with an acquired defect in GLP-1 secretion as well as GIP action (see Diabetes 2007, 56(8), 1951 and Current Diabetes Reports 2006, 6(3), 194). The use of exogenous GLP-1 for treatment of patients with type 2 diabetes is severely limited due to its rapid degradation by the protease DPP-4. Multiple modified peptides have been designed as GLP-1 mimetics that are DPP-4 resistant and show longer half-lives than endogenous GLP-1. Agents with this profile that have been shown to be highly effective for treatment of type 2 diabetes include exenatide and liraglutide, however, these agents require injection. Oral agents that inhibit DPP-4, such as sitagliptin vildagliptin, and saxagliptin, elevate intact GLP-1 and modestly control circulating glucose levels (see Pharmacology & Therapeutics 2010, 125(2), 328; Diabetes Care 2007, 30(6), 1335; Expert Opinion on Emerging Drugs 2008, 13(4), 593). New oral medications that increase GLP-1 secretion would be desirable for treatment of type 2 diabetes.
Bile acids have been shown to enhance peptide secretion from the gastrointestinal tract. Bile acids are released from the gallbladder into the small intestine after each meal to facilitate digestion of nutrients, in particular fat, lipids, and lipid-soluble vitamins. Bile acids also function as hormones that regulate cholesterol homeostasis, energy, and glucose homeostasis via nuclear receptors (FXR, PXR, CAR, VDR) and the G-protein coupled receptor TGR5 (for reviews see: Nature Drug Discovery 2008, 7, 672; Diabetes, Obesity and Metabolism 2008, 10, 1004). TGR5 is a member of the Rhodopsin-like subfamily of GPCRs (Class A) that is expressed in intestine, gall bladder, adipose tissue, liver, and select regions of the central nervous system. TGR5 is activated by multiple bile acids with lithocholic and deoxycholic acids as the most potent activators (Journal of Medicinal Chemistry 2008, 51(6), 1831). Both deoxycholic and lithocholic acids increase GLP-1 secretion from an enteroendocrine STC-1 cell line, in part through TGR5 (Biochemical and Biophysical Research Communications 2005, 329, 386). A synthetic TGR5 agonist INT-777 has been shown to increase intestinal GLP-1 secretion in vivo in mice (Cell Metabolism 2009, 10, 167). Bile salts have been shown to promote secretion of GLP-1 from colonic L cells in a vascularly perfused rat colon model (Journal of Endocrinology 1995, 145(3), 521) as well as GLP-1, peptide YY (PYY), and neurotensin in a vascularly perfused rat ileum model (Endocrinology 1998, 139(9), 3780). In humans, infusion of deoxycholate into the sigmoid colon produces a rapid and marked dose responsive increase in plasma PYY and enteroglucagon concentrations (Gut 1993, 34(9), 1219). Agents that increase ileal and colonic bile acid or bile salt concentrations will increase gut peptide secretion including, but not limited to, GLP-1 and PYY.
Bile acids are synthesized from cholesterol in the liver then undergo conjugation of the carboxylic acid with the amine functionality of taurine and glycine. Conjugated bile acids are secreted into the gall bladder where accumulation occurs until a meal is consumed. Upon eating, the gall bladder contracts and empties its contents into the duodenum, where the conjugated bile acids facilitate absorption of cholesterol, fat, and fat-soluble vitamins in the proximal small intestine (For reviews see: Frontiers in Bioscience 2009, 14, 2584; Clinical Pharmacokinetics 2002, 41(10), 751; Journal of Pediatric Gastroenterology and Nutrition 2001, 32, 407). Conjugated bile acids continue to flow through the small intestine until the distal ileum where 90% are reabsorbed into enterocytes via the apical sodium-dependent bile acid transporter (ASBT, also known as iBAT). The remaining 10% are deconjugated to bile acids by intestinal bacteria in the terminal ileum and colon of which 5% are then passively reabsorbed in the colon and the remaining 5% being excreted in feces. Bile acids that are reabsorbed by ASBT in the ileum are then transported into the portal vein for recirculation to the liver. This highly regulated process, called enterohepatic recirculation, is important for the body's overall maintenance of the total bile acid pool as the amount of bile acid that is synthesized in the liver is equivalent to the amount of bile acids that are excreted in feces. Pharmacological disruption of bile acid reabsorption with an inhibitor of ASBT leads to increased concentrations of bile acids in the colon and feces, a physiological consequence being increased conversion of hepatic cholesterol to bile acids to compensate for fecal loss of bile acids. Many pharmaceutical companies have pursued this mechanism as a strategy for lowering serum cholesterol in patients with dyslipidemia/hypercholesterolemia (For a review see: Current Medicinal Chemistry 2006, 13, 997). Importantly, ASBT-inhibitor mediated increase in colonic bile acid/salt concentration also will increase intestinal GLP-1, PYY, GLP-2, and other gut peptide hormone secretion. Thus, inhibitors of ASBT could be useful for treatment of type 2 diabetes, type 1 diabetes, dyslipidemia, obesity, short bowel syndrome, Chronic Idiopathic Constipation, Irritable bowel syndrome (IBS), Crohn's disease, and arthritis.
Certain 1,4-thiazepines are disclosed, for example in WO 94/18183 and WO 96/05188. These compounds are said to be useful as ileal bile acid reuptake inhibitors (ASBT).